Cobalt based alloy product, method for manufacturing same, and cobalt based alloy article

ABSTRACT

There is provided a cobalt-based alloy product comprising: in mass %, 0.08-0.25% C; 0.1% or less B; 10-30% Cr; 5% or less Fe and 30% or less Ni, the total amount of Fe and Ni being 30% or less; W and/or Mo, the total amount of W and Mo being 5-12%; at least one of Ti, Zr, Hf, V, Nb and Ta, the total amount of Ti, Zr, Hf, V, Nb and Ta being 0.5-2%; 0.5% or less Si; 0.5% or less Mn; 0.003-0.04% N; and the balance being Co and impurities. The cobalt-based alloy product is a polycrystalline body of matrix phase crystal grains, wherein MC type carbide phase grains are dispersively precipitated in the matrix phase crystal grains at an average intergrain distance of 0.13 to 2 μm and M23C6 type carbide phase grains are precipitated on grain boundaries of the matrix phase crystal grains.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to cobalt based alloy materials havingexcellent mechanical properties and, in particular, to a cobalt basedalloy product, a method for manufacturing the same, and a cobalt basedalloy article.

DESCRIPTION OF BACKGROUND ART

Cobalt (Co) based alloy articles, along with nickel (Ni) based alloyarticles, are representative heat resistant alloy materials. Alsoreferred to as super alloys, they are widely used for high temperaturemembers (components used under high temperature environment, e.g. gasturbine members, steam turbine members, etc.). Although Co based alloyarticles are higher in material costs than Ni based alloy articles, theyhave been used for applications such as turbine stator blades andcombustor members because of their excellence in corrosion resistanceand abrasion resistance, and their ease of solid solution strengthening.

In Ni based alloy materials, various improvements that have been made sofar in composition and manufacturing processes of heat resistant alloymaterials have led to the development of strengthening through γ′ phase(e.g. Ni₃(Al, Ti) phase) precipitation, which has now become mainstream.On the other hand, in Co based alloy materials, an intermetalliccompound phase that contributes to improving mechanical properties, likethe γ′ phase in Ni based alloy materials, hardly precipitates, which hasprompted research on carbide phase precipitation strengthening.

For example, Patent Literature 1 (JP Shou 61 (1986)-243143 A) disclosesa Co based superplastic alloy made up of a Co based alloy matrix havinga crystal grain size of equal to or less than 10 μm and carbide grainsin a granular form or a particulate form having a grain size of 0.5 to10 μm precipitated in the matrix. The Co based alloy includes 0.15 to 1wt. % of C, 15 to 40 wt. % of Cr, 3 to 15 wt. % of W or Mo, 1 wt. % orless of B, 0 to 20 wt. % of Ni, 0 to 1.0 wt. % of Nb, 0 to 1.0 wt. % ofZr, 0 to 1.0 wt. % of Ta, 0 to 3 wt. % of Ti, 0 to 3 wt. % of Al, andthe balance of Co. According to Patent Literature 1 (JP Shou 61(1986)-243143 A), there can be provided a Co based superplastic alloythat exhibits superplasticity accompanying with an elongation of equalto or more than 70% even in a low temperature range (e.g. at 950° C.),and is capable of being formed into an article with a complicated shapeby plastic working such as forging.

Patent Literature 2 (JP Hei 7 (1995)-179967 A) discloses a Co basedalloy that is excellent in corrosion resistance, abrasion resistance,and high temperature strength. The alloy includes 21 to 29 wt. % of Cr,15 to 24 wt. % of Mo, 0.5 to 2 wt. % of B, 0.1 or more and less than 0.5wt. % of Si, more than 1 and equal to or less than 2 wt. % of C, 2 wt. %or less of Fe, 2 wt. % or less of Ni, and the balance of substantiallyCo. According to Patent Literature 2 (JP Hei 7 (1995)-179967 A), the Cobased alloy has a composite structure in which a molybdenum boride and achromium carbide are relatively finely dispersed in a quaternary alloyphase of Co, Cr, Mo and Si and exhibits excellent corrosion resistance,abrasion resistance, and high strength.

Meanwhile, in recent years, three dimensional shaping technology (theso-called 3D printing) such as additive manufacturing or AM has receivedmuch attention as a technique for manufacturing finished products with acomplicated shape by near net shaping. To apply the three dimensionalshaping technology to heat resistant alloy components, vigorous researchand development activities are currently being carried out.

For example, Patent Literature 3 (JP 2016-535169 A) discloses a methodof producing layers including the steps of: (a) providing a source of apowdered or suspended granular composite powder having a porosity of 20%or less; (b) depositing a first portion of said powder onto a targetsurface; (c) depositing energy into the powder of said first portionunder conditions that said energy causes sintering, fusing or melting ofthe first powder portion so as to create a first layer; (d) depositing asecond portion of powder onto said first layer; and (e) depositingenergy into the powder of said second portion under conditions that saidenergy causes sintering, fusing or melting of the second powder portionso as to create a second layer. In the method, the energy is supplied bya laser.

Patent Literature 3 (JP 2016-535169 A) teaches as follows: Selectivelaser melting (SLM) or direct metal laser melting (DMLM) uses laser tomake a material a full melt. Full melting is typically useful for amonomaterial (e.g. pure titanium or a single alloy such as Ti-6Al-4V),as there is just one melting point. By contrast, selective lasersintering (SLS) and direct metal laser sintering (DMLS) are essentiallythe same thing, and SLS/DMLS is used to apply processes to a variety ofmaterials multiple metals, alloys, or combinations of alloys and othermaterials such as plastics, ceramics, polymers, carbides or glasses.Meanwhile, sintering is apart from melting, and a sintering process doesnot fully melt a material but heats it to the point that the materialcan fuse together on a molecular level.

CITATION LIST Patent Literature

Patent Literature 1: JP Shou 61 (1986)-243143 A,

Patent Literature 2: JP Hei 7 (1995)-179967 A, and

Patent Literature 3: JP 2016-535169 A.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Since the 3D printing is capable of directly forming even components ofcomplicated shape, manufacturing of turbine high temperature componentsby the 3D printing is very attractive in terms of reduction ofmanufacturing work time and improvement of manufacturing yield (i.e.reduction of manufacturing cost).

Co based alloy materials such as the ones disclosed in PatentLiteratures 1 and 2 are thought to have mechanical properties superiorto those of previous Co based alloy materials. Unfortunately, however,their mechanical properties are inferior to those ofprecipitation-strengthened Ni based alloy materials of recent years.Therefore, many studies on additively manufactured articles (AMarticles) for use as high temperature components are currently directedtoward precipitation-strengthened Ni based alloy materials.

However, AM articles of the precipitation-strengthened Ni based alloysare prone to have problems such as generation of the γ′ phase, which isthe core of their mechanical properties, being hindered and internaldefects occurring in the finished products. As a result, expectedmechanical properties have not been sufficiently achieved. This isattributable to the fact that current precipitation-strengthened Nibased alloy materials used for high temperature components have beenoptimized through melting and forging processes under high vacuum, andtherefore oxidation and nitriding of the Al component and the Ticomponent, which constitute the γ′ phase, easily occur at the stages ofpreparing alloy powder for AM and performing AM.

On the other hand, manufacturing the Co based alloy materials such asthe ones disclosed in Patent Literatures 1 and 2 does not requireprecipitation of an intermetallic compound phase such as the γ′ phase asin Ni based alloy materials, so Co based alloy materials do not containplenty of Al or Ti, which is easily oxidized. This means melting andforging processes in the air atmosphere are available for theirmanufacturing. Therefore, such Co based alloy materials are consideredto be advantageous in manufacturing of alloy powder for AM andmanufacturing of AM articles. Also, the Co based alloy materials haveadvantages with corrosion resistance and abrasion resistance comparableto or superior to those of Ni based alloy materials.

However, as mentioned above, conventional Co based alloy materials havedisadvantages of mechanical properties inferior to those of γ′ phaseprecipitation-strengthened Ni based alloy materials. In other words, ifa Co based alloy material could achieve mechanical properties comparableto or superior to those of γ′ phase precipitation-strengthened Ni basedalloy materials (e.g. a creep rupture time of 1,100 hours or more by acreep test under conditions of a temperature of 900° C. and a stress of98 MPa), AM articles of the Co based alloy material would become highlyattractive high temperature components.

The present invention was made in view of the foregoing and has anobjective to provide a Co based alloy product having mechanicalproperties comparable to or superior to those ofprecipitation-strengthened Ni based alloy materials, and a method formanufacturing the Co based alloy product. Also, another objective of theinvention is to provide an article as a base material for the Co basedalloy product.

Solution to Problems

(I) According to one aspect of the present invention, there is provideda cobalt based alloy product. The cobalt based alloy product has achemical composition including: 0.08 to 0.25 mass % of carbon (C); 0.1mass % or less of boron (B); 10 to 30 mass % of chromium (Cr); 5 mass %or less of iron (Fe) and 30 mass % or less of nickel (Ni), the totalamount of the Fe and the Ni being 30 mass % or less; tungsten (W) and/ormolybdenum (Mo), the total amount of the W and the Mo being 5 to 12 mass%; at least one of titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb) and tantalum (Ta), the total amount of the Ti, the Zr,the Hf, the V, the Nb and the Ta being 0.5 to 2 mass %; 0.5 mass % orless of silicon (Si); 0.5 mass % or less of manganese (Mn); 0.003 to0.04 mass % of nitrogen (N); and the balance being cobalt (Co) andimpurities. The impurities include 0.5 mass % or less of aluminum (Al),and 0.04 mass % or less of oxygen (O). The product is a polycrystallinebody of matrix phase crystal grains. In the polycrystalline body, MCtype carbide phase grains and M₂₃C₆ type carbide phase grains areco-precipitated. The MC type carbide phase grains are dispersivelyprecipitated in the matrix phase crystal grains at an average intergraindistance of 0.13 to 2 μm; and the M₂₃C₆ type carbide phase grains areprecipitated on grain boundaries of the matrix phase crystal grains.

In the above Co based alloy product (I) of the invention, the followingchanges and modifications can be made.

(i) The MC type carbide phase grains may be grains of an MC type carbidephase including the Ti, the Zr, the Hf, the V, the Nb and/or the Ta; andthe M₂₃C₆ type carbide phase grains may be grains of an M₂₃C₆ typecarbide phase including the Cr, the Fe, the W, the Mo and/or the Mn.

(ii) In the case that the chemical composition includes the Ti, contentof the Ti may be 0.01 to 1 mass %; in the case that the chemicalcomposition includes the Zr, content of the Zr may be 0.05 to 1.5 mass%; in the case that the chemical composition includes the Hf, content ofthe Hf may be 0.01 to 0.5 mass %; in the case that the chemicalcomposition includes the V, content of the V may be 0.01 to 0.5 mass %;in the case that the chemical composition includes the Nb, content ofthe Nb may be 0.02 to 1 mass %; and in the case that the chemicalcomposition includes the Ta, content of the Ta may be 0.05 to 1.5 mass%.

(iii) The product may exhibit a creep rupture time of 1,200 hours ormore by a creep test under conditions of a temperature of 850° C. and astress of 168 MPa.

(iv) The product may be a high temperature member.

(v) The high temperature member may be a turbine stator blade, a turbinerotor blade, a turbine combustor nozzle, or a heat exchanger.

(II) According to another aspect of the invention, there is provided amethod for manufacturing the above Co based alloy product. Themanufacturing method includes: an alloy powder preparation step ofpreparing a cobalt based alloy powder having the chemical composition,the cobalt based alloy powder having a predetermined particle size; aselective laser melting step of forming an additively manufacturedarticle, the step comprising alternate repetition of an alloy powder bedpreparation substep of laying the Co based alloy powder such that itforms an alloy powder bed having a predetermined thickness and a lasermelting solidification substep of irradiating a predetermined region ofthe alloy powder bed with a laser beam to locally melt and rapidlysolidify the Co based alloy powder in the region, the laser beam havinga predetermined output power and a predetermined scanning speed; and afirst heat treatment step of subjecting the additively manufacturedarticle to a first heat treatment at a temperature ranging of 750° C. ormore but less than 1,100° C.; and a second heat treatment step ofsubjecting the first heat-treated article to a second heat treatment ata temperature ranging from 600° C. to 1,000° C. but less than thetemperature of the first heat treatment. In the a selective lasermelting step, the predetermined thickness of the alloy powder bed h(unit: μm), the predetermined output power of the laser beam P (unit:W), and the predetermined scanning speed of the laser beam S (unit:mm/s) are controlled to satisfy the following formulas: “15<h<150” and“67×(P/S)−3.5<h<2222×(P/S)+13”.

In the above manufacturing method of the Co based alloy product (II) ofthe invention, the following changes and modifications can be made.

(v) The alloy powder preparation step may include an alloy powderclassification substep of regulating the particle size of the cobaltbased alloy powder to 5 to 100 μm.

Advantages of the Invention

According to the present invention, there can be provided a Co basedalloy product having mechanical properties comparable to or superior tothose of precipitation-strengthened Ni based alloy materials, and amethod for manufacturing the Co based alloy product. Also, there can beprovided an article as a base material for the Co based alloy product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing an exemplary process of a method formanufacturing a Co based alloy product according to an embodiment of thepresent invention;

FIG. 2 is a scanning electron microscope (SEM) image showing anexemplary microstructure of a Co based alloy AM article obtained by aselective laser melting (SLM) step;

FIG. 3 is an SEM image showing an exemplary microstructure of a Co basedalloy AM article obtained through a first heat treatment step;

FIG. 4 is an SEM image showing an exemplary microstructure of a Co basedalloy product obtained through a second heat treatment step;

FIG. 5 is a schematic illustration of a perspective view showing aturbine stator blade which is a Co based alloy product as a hightemperature member according to an embodiment of the invention;

FIG. 6 is a schematic illustration of a cross-sectional view showing agas turbine equipped with a Co based alloy product according to anembodiment of the invention;

FIG. 7 is a schematic illustration of a perspective view showing a heatexchanger which is a Co based alloy product as a high temperature memberaccording to an embodiment of the invention; and

FIG. 8 shows exemplary selective laser melting (SLM) conditions in theSLM step, indicating a relationship between a thickness of an alloypowder bed and a local heat input.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Basic Concept of the Present Invention]

As mentioned before, various research and development activities havebeen carried out on strengthening of Co based alloy materials throughcarbide phase precipitation. Carbide phases that contribute toprecipitation strengthening include, e.g., MC type carbide phases of Ti,Zr, Hf, V, Nb and Ta, and complex carbide phases of these metallicelements.

The C component, which is indispensable in formation of a carbide phasewith each of Ti, Zr, Hf, V, Nb and Ta, tends to segregate significantlyat final solidification portions (e.g. dendrite boundaries, crystalgrain boundaries, etc.) at the melting and solidification stages of theCo based alloy. So, in conventional Co based alloy materials, thecarbide phase grains precipitate along the dendrite boundaries andcrystal grain boundaries in the matrix. In a general cast material of aCo based alloy, for example, the average spacing between dendriteboundaries and the average crystal grain size are on the order of 10¹ to10² μm, and therefore the average spacing between carbide phase grainsis also on the order of 10¹ to 10² μm. Furthermore, even with therelatively fast solidification rate of laser welding, for example, theaverage spacing between carbide phase grains at the solidified portionsis around 5 μm.

Precipitation strengthening in alloys is generally known to be inverselyproportional to the average spacing between precipitates, and it isconsidered that precipitation strengthening is effective only when theaverage spacing between precipitates is around 2 μm or less. However,with the above-mentioned conventional technology, the average spacingbetween precipitates has not reached this level in a Co based alloymaterial, and sufficient precipitation strengthening effect has not beenachieved. In other words, with the conventional technology, it has beendifficult to finely and dispersedly precipitate carbide phase grainsthat might contribute to strengthening alloys. This would be the mainfactor behind the fact that Co based alloy materials have been said tohave mechanical properties inferior to those ofprecipitation-strengthened Ni based alloy materials.

Meanwhile, another carbide phase that can precipitate in Co based alloysis the Cr carbide phase. Since the Cr component is highly solid solubleto the Co based alloy matrix and hardly segregate, the Cr carbide phasecan be dispersedly precipitated in the matrix phase crystal grains.However, the Cr carbide phase is poor in lattice matching with the Cobased alloy matrix crystals and is known to be not so effective as aprecipitation-strengthening phase.

The present inventors thought that if they were able to dispersedlyprecipitate carbide phase grains contributing to precipitationstrengthening in the matrix phase crystal grains, they would be able todramatically improve mechanical properties of Co based alloy materials.Considering the inherent excellent corrosion resistance and abrasionresistance of Co based alloy materials, they would be able to provide aheat-resistant alloy material that would surpassprecipitation-strengthened Ni based alloy materials.

Then, in order to obtain such a Co based alloy material, the inventorsconducted intensive research on alloy compositions and manufacturingmethods. As a result, they have found that segregation cells with asmall size are formed, in which specific components (components formingcarbide phases contributing to alloy strengthening) are segregated, inthe matrix phase crystal grains of a Co based alloy additivelymanufactured article by optimizing the alloy composition and controllingthe amount of heat input for local melting and rapid solidification inan additive manufacturing method (in particular, selective lasermelting). Furthermore, they have found that it is possible todispersedly and finely precipitate MC type carbide phase grains in thematrix phase crystal grains and to precipitate M₂₃C₆ type carbide phasegrains on boundaries of the matrix phase crystal grains by conducting aspecific heat treatment to the obtained AM article. The presentinvention was made based on these findings.

Preferred embodiments of the present invention will be hereinafterdescribed with reference to the accompanying drawings. However, theinvention is not limited to the specific embodiments described below,and various combinations with known art and modifications based on knownart are possible without departing from the spirit and the scope of theinvention.

[Method for Manufacturing Co Based Alloy Product]

FIG. 1 is a flow diagram showing an exemplary process of a method formanufacturing a Co based alloy product according to an embodiment of theinvention. As shown in FIG. 1, the method for manufacturing a Co basedalloy product roughly includes: an alloy powder preparation step S1 ofpreparing a Co based alloy powder; a selective laser melting step S2 offorming the prepared Co based alloy powder into an AM article with adesired shape; a first heat treatment step S3 of conducting a first heattreatment to the AM article formed, and a second heat treatment step S4of conducting a second heat treatment to the AM article conducted withthe first heat treatment.

In addition, although not shown in FIG. 1, a Co based alloy product ofthe invention may be an article that is further applied a corrosionresistant coating formation step and/or a surface finishing step to theCo based alloy product obtained through the second heat treatment stepS4. Also, a Co based alloy powder itself prepared by the alloy powderpreparation step S1 may be an article according to the invention as abase material for the Co based alloy product.

Each step will be hereinafter described in more detail.

(Alloy Powder Preparation Step)

In the step S1, a Co based alloy powder having a predetermined chemicalcomposition is prepared. The chemical composition preferably includes:0.08 to 0.25 mass % of C; 0.1 mass % or less of B; 10 to 30 mass % ofCr; 5 mass % or less of Fe and 30 mass % or less of Ni, the total amountof the Fe and the Ni being 30 mass % or less; W and/or Mo, the totalamount of the W and the Mo being 5 to 12 mass %; at least one of Ti, Zr,Hf, V, Nb and Ta, the total amount of the Ti, the Zr, the Hf, the V, theNb and the Ta being 0.5 to 2 mass %; 0.5 mass % or less of Si; 0.5 mass% or less of Mn; 0.003 to 0.04 mass % of N; and the balance being Co andimpurities. As impurities, 0.5 mass % or less of Al and 0.04 mass % orless of O may be included.

C: 0.08 to 0.25 Mass %

The C component is an important component that constitutes an MC typecarbide phase to serve as a precipitation strengthening phase (this MCtype carbide phase may be referred to as “carbide phase of Ti, Zr, Hf,V, Nb and/or Ta” or “precipitation reinforcing carbide phase”). Also,the C component is an important component that constitutes an M₂₃C₆ typecarbide phase to serve as a grain boundary slip suppressing phase of thematrix phase crystal grains (this M₂₃C₆ type carbide phase may bereferred to as “carbide phase of Cr, Fe, W, Mo and/or Mn” or “grainboundary strengthening carbide phase”). The content of the C componentis preferably 0.08 to 0.25 mass %, more preferably 0.1 to 0.2 mass %,and even more preferably 0.12 to 0.18 mass %. When the C content is lessthan 0.08 mass %, the amount of precipitation of the precipitationreinforcing carbide phase and the grain boundary strengthening carbidephase is insufficient, resulting in an insufficient effect of improvingthe mechanical properties. By contrast, when the C content is over 0.25mass %, carbide phases other than the MC type carbide phase precipitateexcessively, and/or the alloy material becomes excessively hard, whichleads to deteriorated ductility and toughness.

B: 0.1 Mass % or Less

The B component contributes to improving bondability between crystalgrain boundaries (the so-called grain boundary strengthening). Althoughthe B is not an essential component, when it is contained in the alloy,the content of the B component is preferably 0.1 mass % or less and morepreferably 0.005 to 0.05 mass %. When the B component is over 0.1 mass%, cracking (e.g. solidification cracking) is prone to occur duringformation of the AM article.

Cr: 10 to 30 Mass %

The Cr component contributes to improving corrosion resistance andoxidation resistance, and is a component that constitutes the grainboundary strengthening carbide phase. The content of the Cr component ispreferably 10 to 30 mass % and more preferably 15 to 27 mass %. In thecase where a corrosion resistant coating layer is provided on theoutermost surface of the Co based alloy product, the content of the Crcomponent is even more preferably 10 to 18 mass %. When the Cr contentis less than 10 mass %, advantageous effects such as improvements of thecorrosion resistance and the oxidation resistance and generation of thegrain boundary strengthening carbide phase are insufficient. When the Crcontent is over 30 mass %, the brittle σ phase and/or the excessiveamount of Cr carbide phase are generated, resulting in deterioratedmechanical properties (i.e. toughness, ductility, strength, etc.).

Ni: 30 Mass % or Less

Being similar to Co component in properties but less expensive than Co,the Ni component may be used to replace part of the Co component.Although the Ni is not an essential component, when it is contained inthe alloy, the content of the Ni component is preferably 30 mass % orless, more preferably 20 mass % or less, and more preferably 5 to 15mass %. When the Ni content is over 30 mass %, the abrasion resistanceand the local stress resistance, which are characteristics of Co basedalloys, deteriorate. This is attributable to the difference in stackingfault energy between Co and Ni.

Fe: 5 Mass % or Less

Being much less expensive than Ni and similar to Ni component inproperties, the Fe component may be used to replace part of the Nicomponent. In addition, the Fe component is a component that canconstitute the grain boundary strengthening carbide phase. The totalcontent of the Fe and Ni is preferably 30 mass % or less, morepreferably 20 mass % or less, and even more preferably 5 to 15 mass %.Although the Fe is not an essential component, when it is contained inthe alloy, the content of the Fe component is preferably 5 mass % orless and more preferably 3 mass % or less in the range less than the Nicontent. When the Fe content is over 5 mass %, the corrosion resistanceand mechanical properties deteriorate.

W and/or Mo: 5 to 12 Mass % in Total

The W component and the Mo component contribute tosolution-strengthening the matrix. Furthermore, part of the W and Mocomponents may constitute the grain boundary strengthening carbidephase. The total content of the W component and/or the Mo component (atleast one of W and Mo components) is preferably 5 to 12 mass % and morepreferably 7 to 10 mass %. When the total content of the W component andthe Mo component is less than 5 mass %, the solution strengthening ofthe matrix is insufficient. In contrast, when the total content of the Wcomponent and the Mo component is over 12 mass %, the brittle σ phasetends to be generated easily, resulting in deteriorated mechanicalproperties (i.e. toughness, ductility, etc.).

Re: 2 Mass % or Less

The Re component contributes to solution-strengthening the matrix andimproving corrosion resistance. Although the Re is not an essentialcomponent, when it is contained in the alloy to replace part of the Wcomponent or the Mo component, the content of the Re component ispreferably 2 mass % or less and more preferably 0.5 to 1.5 mass %. Whenthe Re content is over 2 mass %, the advantageous effects of the Recomponent become saturated, and the material costs become too high.

Ti, Zr, Hf, V, Nb and Ta: 0.5 to 2 Mass % in Total

The Ti component, the Zr component, the Hf component, the V component,the Nb component, and the Ta component are important components thatconstitute the precipitation reinforcing carbide phase (the MC typecarbide phase), and at least one of these components are preferablyincluded. The total content of the Ti, Zr, Hf, V, Nb and Ta componentsis preferably 0.5 to 2 mass % and more preferably 0.5 to 1.8 mass %.When the total content is less than 0.5 mass %, the amount ofprecipitation of the precipitation reinforcing carbide phase isinsufficient, and, as a result, the effect of improving the mechanicalproperties is insufficient. In contrast, when the total content is over2 mass %, the mechanical properties deteriorate due to coarsening of thegrains of the precipitation reinforcing carbide phase, acceleratedgeneration of a brittle phase (e.g. σ phase), generation of grains of anoxide phase that does not contribute to precipitation strengthening,etc.

More specifically, when the Ti component is included, the Ti content ispreferably 0.01 to 1 mass % and more preferably 0.05 to 0.8 mass %.

When the Zr component is included, the Zr content is preferably 0.05 to1.5 mass % and more preferably 0.1 to 1.2 mass %. From the viewpoint ofthe mechanical strength, it is preferable that the Zr component isincluded in the alloy.

When the Hf component is included, the Hf content is preferably 0.01 to0.5 mass % and more preferably 0.02 to 0.1 mass %.

When the V component is included, the V content is preferably 0.01 to0.5 mass % and more preferably 0.02 to 0.1 mass %.

When the Nb component is included, the Nb content is preferably 0.02 to1 mass % and more preferably 0.05 to 0.8 mass %.

When the Ta component is included, the Ta content is preferably 0.05 to1.5 mass % and more preferably 0.1 to 1.2 mass %.

Si: 0.5 Mass % or Less

The Si component serves as a deoxidant agent and contributes toimproving the mechanical properties. Although the Si is not an essentialcomponent, when it is contained in the alloy, the content of the Sicomponent is preferably 0.5 mass % or less and more preferably 0.01 to0.3 mass %. When the Si content is over 0.5 mass %, coarse grains of anoxide (e.g. SiO₂) are generated, which causes deterioration of themechanical properties.

Mn: 0.5 Mass % or Less

The Mn component serves as a deoxidant agent and a desulfurizing agentand contributes to improving the mechanical properties and the corrosionresistance. In addition, the Mn component is a component that mayconstitute the grain boundary strengthening carbide phase. Although theMn is not an essential component, when it is contained in the alloy, thecontent of the Mn component is preferably 0.5 mass % or less and morepreferably 0.01 to 0.3 mass %. When the Mn content is over 0.5 mass %,coarse grains of a sulfide (e.g. MnS) are generated, which causesdeterioration of the mechanical properties and the corrosion resistance.

N: 0.003 to 0.04 Mass %

The N component contributes to stabilizing the generation of theprecipitation reinforcing carbide phase. The content of the N componentis preferably 0.003 to 0.04 mass %, more preferably 0.005 to 0.03 mass%, and even more preferably 0.007 to 0.025 mass %. When the N content isless than 0.003 mass %, the advantageous effects of the N component areinsufficient. Meanwhile, when the N content is over 0.04 mass %, coarsegrains of a nitride (e.g. Cr nitride) are generated, which causesdeterioration of the mechanical properties.

Balance: Co Component and Impurities

The Co component is one of the key components of the alloy and itscontent is the largest of all the components. As mentioned above, Cobased alloy materials have the advantages of having corrosion resistanceand abrasion resistance comparable to or superior to those of Ni basedalloy materials.

The Al component is one of the impurities of the alloy and is not to beintentionally included in the alloy. However, an Al content of 0.5 mass% or less is acceptable as it does not have any serious negativeinfluence on the mechanical properties of the Co based alloy product.When the Al content is over 0.5 mass %, coarse grains of an oxide ornitride (e.g. Al₂O₃ or AlN) are generated, which causes deterioration ofthe mechanical properties.

The O component is also one of the impurities of the alloy and is not tobe intentionally included in the alloy. However, an O content of 0.04mass % or less is acceptable as it does not have any serious negativeinfluence on the mechanical properties of the Co based alloy product.When the O content is over 0.04 mass %, coarse grains of each oxide(e.g. Ti oxide, Zr oxide, Al oxide, Fe oxide, Si oxide, etc.) aregenerated, which causes deterioration of the mechanical properties.

In this step S1, there is no particular limitation on the method andtechniques for preparing the Co based alloy powder, and any conventionalmethod and technique may be used. For example, a master ingotmanufacturing substep S1 a of manufacturing a master ingot by mixing,melting, and casting the raw materials such that the ingot has a desiredchemical composition and an atomization substep S1 b of forming thealloy powder from the master ingot may be performed. Also, there is noparticular limitation on the atomization method, and any conventionalmethod and technique may be used. For example, gas atomizing orcentrifugal force atomizing, by which spherical particles of high puritycan be obtained, may be preferably used.

For ease of handling and ease of filling the alloy powder bed in thefollowing selective laser melting step S2, the particle size of thealloy powder is preferably 5 to 100 μm, more preferably 10 to 70 μm, andeven more preferably 10 to 50 μm. When the particle size of the alloypowder is less than 5 μm, fluidity of the alloy powder decreases in thefollowing step S2 (i.e. formability of the alloy powder bed decreases),which causes deterioration of shape accuracy of the AM article. Incontrast, when the particle size of the alloy powder is over 100 μm,controlling the local melting and rapid solidification of the alloypowder bed in the following step S2 becomes difficult, which leads toinsufficient melting of the alloy powder and increased surface roughnessof the AM article.

In view of the above, an alloy powder classification substep S1 c ispreferably performed so as to regulate the alloy powder particle size to5 to 100 μm. In the present invention, when the particle sizedistribution of the alloy powder manufactured in the atomization substepS1 b is observed to fall within the desired range, it is assumed thatthe substep S1 c has been performed.

(Selective Laser Melting Step)

In the step S2, the prepared Co based alloy powder is formed into an AMarticle having a desired shape by selective laser melting (SLM). Morespecifically, this step comprises alternate repetition of an alloypowder bed preparation substep S2 a and a laser melting solidificationsubstep S2 b. In the step S2 a, the Co based alloy powder is laid suchthat it forms an alloy powder bed having a predetermined thickness, andin the step S2 b, a predetermined region of the alloy powder bed isirradiated with a laser beam to locally melt and rapidly solidify the Cobased alloy powder in the region.

In this step S2, in order to obtain a finished Co based alloy producthaving a desired microstructure (a microstructure in which theprecipitation reinforcing carbide phase grains are dispersedlyprecipitated in the matrix phase crystal grains), the microstructure ofthe AM article, which is a precursor of the finished product, iscontrolled by controlling the local melting and the rapid solidificationof the alloy powder bed.

More specifically, the thickness of the alloy powder bed h (unit: μm),the output power of the laser beam P (unit: W), and the scanning speedof the laser beam S (unit: mm/s) are preferably controlled to satisfythe following formulas: “15<h<150” and “67×(P/S)−3.5<h<2222×(P/S)+13”.When these formulas are not satisfied, an AM article having a desiredmicrostructure cannot be obtained.

While the output power P and the scanning speed S of the laser beambasically depend on configurations of the laser apparatus, they may bedetermined so as to satisfy the following formulas: “10≤P≤1000” and“10≤S≤7000”.

(Co Based Alloy Additively Manufactured Article)

FIG. 2 is a scanning electron microscope (SEM) image showing anexemplary microstructure of a Co based alloy AM article obtained by theSLM step S2. As shown in FIG. 2, the Co based alloy AM article obtainedby the SLM step S2 has a very unique microstructure that has never beenseen before.

The AM article is a polycrystalline body of matrix phase crystal grains.In the matrix phase crystal grains of the polycrystalline body,segregation cells with an average size of 0.13 to 2 μm are formed. Inthe viewpoint of the mechanical strength, segregation cells with anaverage size of 0.15 to 1.5 μm are more preferable. It may be recognizedthat the carbide phase grains are precipitated on a part of boundaryregions of the segregation cells. In addition, from various experimentsby the inventors, it can be recognized that the matrix phase crystalgrains with an average size of 5 to 150 μm are preferable.

In the present invention, the size of segregation cells is basicallydefined as the average of the long diameter and the short diameter.However, when an aspect ratio of the longer diameter and the shortdiameter is three or more, twice the short diameter may be adopted asthe size of segregation cell. Also, average distance among grains of theprecipitation reinforcing carbide phase may be defined as the size ofsegregation cells because the precipitation reinforcing carbide phasegrains are precipitated on the boundary regions of the segregationcells.

A more detailed microstructure observation by scanning transmissionelectron microscopy-energy dispersive X-ray spectrometry (STEM-EDX) hasrevealed that the components constituting the precipitation reinforcingcarbide phase (Ti, Zr, Hf, V, Nb, Ta, and C) segregate in the boundaryregions between the neighboring segregation cells (i.e. in outerperipheral regions of micro-cells, similar to cell walls). It has alsobeen observed that grains precipitating on the boundary regions amongthese segregation cells are MC type carbide phase grains.

Naturally enough, the segregation of the components constituting the MCtype carbide phase and the precipitation of the MC type carbide phasegrains can be observed also on the grain boundaries of the matrix phasecrystal grains in the AM article.

(First Heat Treatment Step)

In the step S3, the formed Co based alloy AM article is subjected to afirst heat treatment. The first heat treatment is preferably performedat in a temperature range of 750° C. or more and less than 1,100° C.,more preferably of 800° C. or more and 1,050° C. or less, still morepreferably of 850° C. or more and 1,000° C. or less. With respect to aholding duration, it may be controlled in a time range of 0.5 to 10hours taking the heat treatment temperature into consideration. There isno particular limitation on a cooling method after the heat treatment,and oil cooling, water cooling, air cooling, or furnace cooling may beused.

FIG. 3 is an SEM image showing an exemplary microstructure of a Co basedalloy AM article obtained through the first heat treatment step S3. Asshown in FIG. 3, also the Co based alloy AM article conducted with thefirst heat treatment has a unique microstructure that has never beenseen before.

Interestingly enough, by conducting the first heat treatment, it hasbeen found that the components segregated in the boundary regions of thesegregation cells start to diffuse and combine to form the precipitationreinforcing carbide phase (the MC type carbide phase) on/along theboundary regions, and as a result, cell walls of the segregation cellsalmost disappear (more specifically, it becomes difficult to observe thecell walls of segregation cells by a microstructure observation). Inother words, grains of the precipitation reinforcing carbide phase areformed dispersedly along regions where the cell walls seem to haveexisted (on the boundary regions of ex-segregation cells). In thepresent invention, a region surrounded with the MC type carbide phasegrains precipitating along a region where the cell wall seems to haveexisted is referred to as a “post-segregation cell”.

It can be considered that a post-segregation cell has almost the sameouter form as the segregation cell, and thus an average size ofpost-segregation cells is in a range of 0.13 to 2 μm. Meanwhile, becausethe precipitation reinforcing carbide phase grains can act a role aspinning points against grain boundary migration of the matrix phasecrystal grains, coarsening of the matrix phase crystal grains issuppressed.

Furthermore, it has been found that the grain boundary strengtheningcarbide phase (the M₂₃C₆ type carbide phase) starts to generate on thegrain boundaries of matrix phase crystal grains, in addition to thegeneration of the precipitation reinforcing carbide phase.

Moreover, by the first heat treatment, residual internal strain of theAM article possibly caused by rapid solidification in the SLM step S2can be relaxed, and undesired deformation of the alloy product can berestrained during a latter step and using as a high temperature member.

(Second Heat Treatment Step)

In the step S4, the Co alloy AM articles conducted with the first heattreatment is subjected to a second heat treatment. The second heattreatment is preferably performed at a temperature ranging from 600 to1,000° C. but less than the temperature of the first heat treatment.With respect to a holding duration, it may be controlled in a time rangeof 0.5 to 20 hours taking the heat treatment temperature intoconsideration. There is no particular limitation on the cooling methodafter the second heat treatment, and oil cooling, water cooling, aircooling, or furnace cooling may be used.

FIG. 4 is an SEM image showing an exemplary microstructure of a Co basedalloy product obtained through the second heat treatment step S4. Asshown in FIG. 4, also the Co based alloy product conducted with thesecond heat treatment has a unique microstructure that has never beenseen before. By performing the second heat treatment, the precipitationreinforcing carbide phase grains and the grain boundary strengtheningcarbide phase grains can be appropriately grown while suppressingcoarsening of the matrix phase crystal grains.

As results of analysis with STEM-EDX, it has been confirmed that theprecipitation reinforcing carbide phase can be regarded as an MC typecarbide phase including the Ti, Zr, Hf, V, Nb and/or Ta, and the grainboundary strengthening carbide phase can be regarded as an M₂₃C₆ typecarbide phase including the Cr, Fe, W, Mo and/or Mn.

The Co based alloy product obtained through the step S4 has an averagesize of the matrix phase crystal grains of 5 to 150 μm, in which theprecipitation reinforcing carbide phase grains precipitate dispersedlywith an average intergrain distance of 0.13 to 2 μm in the matrix phasecrystal grains, and the grain boundary strengthening carbide phasegrains precipitate on the grain boundaries of matrix phase crystalgrains. Also, the grains of precipitation reinforcing carbide phase aredispersedly precipitated also on the grain boundaries of matrix phasecrystal grains in the Co based alloy product according to the invention,as described before.

[Co Based Alloy Product]

FIG. 5 is a schematic illustration of a perspective view showing aturbine stator blade which is a Co based alloy product as a hightemperature member according to an embodiment of the invention. As shownin FIG. 5, the turbine stator blade 100 includes an inner ring side endwall 101, a blade part 102, and an outer ring side end wall 103. Insidethe blade part 102 is often formed a cooling structure. As seen fromFIG. 5, since the turbine stator blade 100 has a very complicated shapeand structure, the technical significance of AM articles manufactured bynear net shaping and alloy products based on the AM articles isprofound.

Meanwhile, in the case of a gas turbine for power generation with anoutput of around 30 MW, the length of the blade part 102 of the turbinestator blade 100 (i.e. distance between the end walls 101 and 103) isapproximately 170 mm. Naturally enough, a Co based alloy product of theinvention can be used as a turbine rotor blade.

FIG. 6 is a schematic illustration of a cross-sectional view showing agas turbine equipped with a Co based alloy product according to anembodiment of the invention. As shown in FIG. 6, the gas turbine 200roughly includes a compression part 210 for compressing intake air and aturbine part 220 for blowing combustion gas of a fuel on turbine bladesto obtain rotation power. The high temperature member according to theembodiment of the invention can be preferably used as a turbine nozzle221 or the turbine stator blade 100 inside the turbine part 220.

Also, the high temperature member according to the embodiment of theinvention is not limited to gas turbine applications but may be used forother turbine applications (e.g. steam turbines) and component usedunder high temperature environment in other machines/apparatuses.

FIG. 7 is a schematic illustration of a perspective view showing a heatexchanger which is a Co based alloy product as a high temperature memberaccording to an embodiment of the invention. A heat exchanger 300 shownin FIG. 6 is an example of a plate-fin type heat exchanger, and has abasic structure in which a separation layer 301 and a fin layer 302 arealternatively stacked each other. Both ends in the width direction offlow channels in the fin layer 302 are sealed by a side bar portion 303.Heat exchanging between high temperature fluid and low temperature fluidcan be done by flowing the high temperature fluid and the lowtemperature fluid alternately into adjacent fin layers 302 via theseparation layer 301.

A heat exchanger 300 according to an embodiment of the invention isformed integrally without soldering joining or welding joining theconventional parts constituting a heat exchanger such as separationplates, corrugated fins and side bars. Consequently, the heat exchanger300 has advantages improving heat resistance and weight reduction thanthe conventional heat exchangers. In addition, the heat transferefficiency can be higher by forming an appropriate concavo-convexpattern on the surfaces of the flow channels and making the fluid intoturbulence. Improving the heat transfer efficiency leads to downsizingof the heat exchanger.

EXAMPLES

The present invention will be hereinafter described in more detail withexamples and comparative examples. It should be noted that the inventionis not limited to these examples.

Experimental 1

(Preparation of Alloy Powders IA-1 to IA-7 and CA-1 to CA-5)

Co based alloy powders having the chemical compositions shown in Table 1were prepared (the alloy powder preparation Step S1). Specifically,first, the master ingot manufacturing substep S1 a was performed, inwhich the raw materials were mixed and subjected to melting and castingby a vacuum high frequency induction melting method so as to form amaster ingot (weight: approximately 2 kg) for each powder. Next, theatomization substep S1 b was performed to form each alloy powder. In thesubstep S1 b, each master ingot was remelted and subjected to gasatomizing in an argon gas atmosphere.

Then, each alloy powder thus obtained was subjected to the alloy powderclassification substep S1 c to control the particle size of alloypowder. Each alloy powder was classified into an alloy powder with aparticle size of 5 to 25 μm.

TABLE 1 Chemical Compositions of Alloy Powders IA-1 to IA-7 and CA-1 toCA-5. Chemical Composition (mass %) Ti + Zr + Alloy Hf + V + Powder C BCr Ni Fe W Ti Zr Hf V Nb Ta Si Mn N Co Al O Nb + Ta IA-1 0.16 0.009 24.79.3 0.01 7.5 0.16 0.45 — — 0.20 0.15 0.01 0.01 0.005 Bal. 0.01 0.0050.96 IA-2 0.25 0.011 26.5 10.5 0.90 7.4 0.30 0.60 — — 0.15 0.40 0.300.20 0.030 Bal. 0.05 0.020 1.45 IA-3 0.08 0.009 30.0 — — 5.0 — 0.35 — —0.16 — 0.05 0.01 0.005 Bal. — 0.005 0.51 IA-4 0.10 0.010 25.0 8.0 0.027.5 0.25 0.05 — — 0.09 0.30 0.01 0.02 0.010 Bal. — 0.010 0.69 IA-5 0.180.009 24.9 9.2 0.01 7.6 0.17 0.45 0.02 0.04 0.21 0.16 0.01 0.01 0.015Bal. 0.01 0.010 1.05 IA-6 0.24 0.011 25.5 10.3 0.90 7.4 0.20 0.60 0.050.02 0.15 0.40 0.30 0.20 0.040 Bal. 0.06 0.025 1.42 IA-7 0.08 0.009 29.5— — 6.0 0.10 0.15 0.01 0.04 — 0.30 0.15 0.10 0.005 Bal. — 0.005 0.60CA-1 0.35 0.009 32.5 9.5 0.01 7.3 0.15 0.40 — — 0.05 0.50 0.01 0.010.005 Bal. 0.01 0.005 1.10 CA-2 0.35 0.009 30.0 40.0 0.01 7.3 0.90 0.40— — 1.0  1.0  0.01 0.01 0.005 Bal. 2.20 0.005 3.30 CA-3 0.40 0.010 29.010.0 0.20 7.5 0.20 0.10 — — 0.10 — 0.10 0.02 0.001 Bal. — 0.015 0.40CA-4 0.25 0.010 29.0 10.0 0.10 7.0 — — — — — — — 0.01 0.010 Bal. — 0.0100 CA-5 0.11 0.002 22.0 23.0 0.01 14.0 0.01 0.01 — — — — 0.50 0.003 0.006Bal. 0.01 0.008 0.02 “—” indicates that the element was notintentionally included or not detected. “Bal.” indicates inclusion ofimpurities other than Al and O.

As shown in Table 1, the inventive alloy powders IA-1 to IA-7 havechemical compositions that satisfy the specifications of the invention.In contrast, the comparative alloy powder CA-1 has a C content and a Crcontent that fail to satisfy the specifications of the invention. Thecomparative alloy powder CA-2 has a C content, an Ni content, and atotal content of “Ti+Zr+Hf+V+Nb+Ta” that are out of the specificationsof the invention. The comparative alloy powder CA-3 has a C content, anN content and a total content of “Ti+Zr+Hf+V+Nb+Ta” that are outside ofthe specifications of the invention. The comparative alloy powder CA-4has a total content of “Ti+Zr+Hf+V+Nb+Ta” that fail to satisfy thespecifications of the invention. The comparative alloy powder CA-5 has aW content and a total content of “Ti+Zr+Hf+V+Nb+Ta” that are out of thespecifications of the invention.

[Experiment 2]

(Examination of SLM Conditions in Selective Laser Melting Step)

AM articles (8 mm in diameter×10 mm in length) were formed of the alloypowders IA-4 prepared in Experimental 1 by the SLM process (selectivelaser melting step S2). The output power of the laser beam P was set at85 W, and the local heat input P/S (unit: W×s/mm=J/mm) was controlled byvarying the thickness of the alloy powder bed h and the scanning speed(mm/s) of the laser beam S. Controlling the local heat input correspondsto controlling the cooling rate.

The AM articles formed above were each subjected to microstructureobservation to measure the average segregation cell size. Themicrostructure observation was performed by SEM. Also, the obtained SEMimages were subjected to image analysis using an image processingprogram (ImageJ, a public domain program developed at the NationalInstitutes of Health in U.S.A., or NIH) to measure the average size ofsegregation cells.

FIG. 8 shows exemplary SLM conditions in the SLM step S2, indicating arelationship between the thickness of the alloy powder bed and the localheat input. In FIG. 8, “o” signifies the AM articles observed to have anaverage segregation cell size within a range of 0.15 to 1.5 μm andjudged as “Passed”, and “x” signifies the other AM articles, judged as“Failed”.

Based on the results of Experimental 2, it has been confirmed that inthe selective laser melting step S2, the SLM process is preferablyperformed while controlling the thickness of the alloy powder bed h(unit: μm), the output power of the laser beam P (unit: W), and thescanning speed of the laser beam S (unit: mm/s) such that they satisfythe following formulas: “15<h<150” and “67×(P/S)−3.5<h<2222×(P/S)+13”.In other words, the hatched region is the passed region.

[Experimental 3]

(Examination of Heat Treatment Conditions in First Heat Treatment Step)

AM articles (10 mm in diameter×50 mm in length) were formed of the alloypowders IA-5 and IA-6 prepared in Experimental 1 by the SLM method (theselective laser melting step S2). The thickness of each alloy powder bedh and the output power of the laser beam P were set at 100 μm and 100 W,respectively. The local heat input P/S (unit: W×s/mm=J/mm) wascontrolled by varying the scanning speed (mm/s) of the laser beam S soas to satisfy the passing conditions examined in Experimental 2.

Each AM article formed above was subjected to heat treatment in atemperature range of 950 to 1,250° C. with a holding duration range of 2to 4 hours (the first heat treatment step S3). Next, each AM articleconducted with the first heat treatment was subjected to heat treatmentat 900° C. with a holding duration of 4 hours (the second heat treatmentstep S4) to manufacture a Co based alloy product (IAP-5a to IAP-5d, andIAP-6a to IAP-6d) formed of the powder IA-5 or the powder IA-6. Testpieces for mechanical properties testing were taken from the Co basedalloy products and subjected to mechanical properties testing.

As to the mechanical properties testing, a creep test was conducted at850° C. under a stress of 168 MPa to measure the creep rupture time.Based on the prescribed properties of high temperature members towardwhich the invention is directed, any creep rupture time of 1,200 hoursor more was judged as “Passed”, and any creep rupture time of less than1,200 hours was judged as “Failed”. Such creep properties can be deemedas comparable to those of Ni based alloy materials.

The results of Experimental 3 are shown in Table 2.

TABLE 2 First Heat Treatment Temperature and Creep Testing Results ofAlloy Products IAP-5a to IAP-5d and IAP-6a to IAP-6d. First HeatTreatment Alloy Alloy Temperature Product No. Powder No. (° C.) CreepTest IAP-5a IA-5 1250 Failed IAP-5b 1150 Failed IAP-5c 1050 PassedIAP-5d 950 Passed IAP-6a IA-6 1250 Failed IAP-6b 1150 Failed IAP-6c 1050Passed IAP-6d 950 Passed

As shown in Table 2, the test pieces heat treated at 1,100° C. or morefailed the mechanical properties. In contrast, the test pieces heattreated at lower than 1,100° C. all passed the mechanical properties.This is because the precipitation reinforcing carbide phase grains (theMC type carbide phase grains) and the grain boundary strengtheningcarbide phase grains (the M₂₃C₆ type carbide phase grains) aredispersedly precipitated with a favorable configuration, in addition tothe matrix phase crystal grains with an appropriate average grain size.

[Experimental 4]

(Manufacturing of Alloy Products IAP-1-1 to IAP-7-1 Formed of PowdersIA-1 to IA-5 and Alloy Products CAP-1-1 to CAP-5-1 Formed of PowdersCA-1 to CA-5)

An AM article (10 mm in diameter×50 mm in length) was formed of each ofthe alloy powders IA-1 to IA-7 and CA-1 to CA-5 prepared in Experimental1 by the SLM process (the selective laser melting step S2).

Each AM article formed above was subjected to heat treatment at 1,000°C. with a holding duration of 4 hours (the first heat treatment stepS3). Next, each AM article conducted with the first heat treatment wassubjected to heat treatment at 850° C. with a holding duration of 1 hour(the second heat treatment step S4) to manufacture Co based alloyproducts IAP-1-1 to IAP-7-1 formed of the powders IA-1 to IA-7 and Cobased alloy products CAP-1-1 to CAP-5-1 formed of the powders CA-1 toCA-5.

(Microstructure Observation and Mechanical Properties Testing)

Test pieces for microstructure observation and mechanical propertiestesting were taken from the Co based alloy products IAP-1-1 to IAP-7-1and CAP-1-1 to CAP-5-1 and subjected to microstructure observation andmechanical properties testing.

The microstructure observation was performed by SEM and through imageanalysis of SEM images thereof in a similar manner to Experimental 2 tocheck for co-presence of the precipitation reinforcing carbide phasegrains and the grain boundary strengthening carbide phase grains, and tomeasure the average intergrain distance of precipitation reinforcingcarbide phase grains in the matrix phase crystal grains. Also, themechanical properties test was performed by the creep test in a similarmanner to Experimental 3, and passed/failed for each of the test pieceswas judged based on the same criterion in Experimental 3.

The results of Experimental 4 are shown in Table 3.

TABLE 3 Measurement and Testing Results of Alloy Products IAP-1-1 toIAP-7-1and CAP-1-1 to CAP-5-1. Co-presence of Precipitation AverageIntergrain Reinforcing Carbide Phase Distance of Precipitation Grainsand Grain Boundary Reinforcing Carbide Alloy Alloy Strengthening CarbidePhase Grains Product No. Powder No. Phase Grains (μm) Creep Test IAP-1-1IA-1 Co-precipitated 0.55 Passed IAP-2-1 IA-2 Co-precipitated 0.45Passed IAP-3-1 IA-3 Co-precipitated 1.5  Passed IAP-4-1 IA-4Co-precipitated 0.65 Passed IAP-5-1 IA-5 Co-precipitated 0.55 PassedIAP-6-1 IA-6 Co-precipitated 0.15 Passed IAP-7-1 IA-7 Co-precipitated0.80 Passed CAP-1-1 CA-1 Co-precipitated 0.55 Failed CAP-2-1 CA-2Co-precipitated 2.8  Failed CAP-3-1 CA-3 Co-precipitated 2.5  FailedCAP-4-1 CA-4 Not Precipitated 120*⁾   Failed CAP-5-1 CA-5 NotPrecipitated 100*⁾   Failed *⁾Average size of the matrix phase crystalgrains is entered because the precipitation reinforcing carbide phasegrains were not formed.

As shown in Table 3, it is confirmed that the inventive alloy productsIAP-1-1 to IAP-7-1 according to the invention have microstructures inthat the precipitation reinforcing carbide phase grains and the grainboundary strengthening carbide phase grains are co-precipitated, andthat the average intergrain distance of precipitation reinforcingcarbide phase grains in the matrix phase crystal grains is within arange of 0.15 to 1.5 μm. Also, it is recognized that all of these testpieces pass the mechanical properties.

On the other hand, the comparative alloy products CAP-1-1 to CAP-5-1 allfailed the creep testing, although the precipitation reinforcing carbidephase grains and the grain boundary strengthening carbide phase grainsare co-precipitated in CAP-1-1 to CAP-3-1. To determine possible causesbehind this, the comparative alloy products CAP-1-1 to CAP-5-1 wereexamined individually.

Regarding CAP-1-1, the excessive contents of C and Cr have resulted inexcessive precipitation of Cr carbide grains. Regarding CAP-2-1, theexcessive content of C and the excessive total content of“Ti+Zr+Hf+V+Nb+Ta” have resulted in coarsening of the precipitationreinforcing carbide phase grains and an increased average intergraindistance of the precipitation reinforcing carbide phase grains.Regarding CAP-3-1, the excessive content of C, the insufficient contentof N and the insufficient total content of “Ti+Zr+Hf+V+Nb+Ta” haveresulted in excessive precipitation of the Cr carbide grains and anincreased average intergrain distance of the precipitation reinforcingcarbide phase grains. These results confirm that excessive precipitationof Cr carbide grains and/or an increased average intergrain distance ofthe precipitation reinforcing carbide phase grains lead to insufficientmechanical properties.

Regarding CAP-4-1 and CAP-5-1, insufficient total content of“Ti+Zr+Hf+V+Nb+Ta” (almost none) has resulted in no formation of thesegregation cells in the AM articles through the selective laser meltingstep S2 and no formation of the precipitation reinforcing carbide phasegrains in the matrix phase crystal grains through the first heattreatment step S3 and the second heat treatment step S4. Therefore, itcan be considered that no formation of the precipitation reinforcingcarbide phase grains in the matrix phase crystal grains has led tofailure in the mechanical properties.

Based on the results of Experimental 4, it has been confirmed that IA-1to IA-7, which have the chemical compositions specified in theinvention, are desirable as starting materials for Co based alloyproducts. It has also been confirmed that dispersed formation of theprecipitation reinforcing carbide phase grains within a range of 0.15 to1.5 μm in the matrix phase crystal grains and precipitation of the grainboundary strengthening carbide phase grains on the grain boundaries ofmatrix phase crystal grains have led to improving the creep propertiesof the Co based alloy products.

The above-described embodiments and Examples have been specificallygiven in order to help with understanding on the present invention, butthe invention is not limited to the described embodiments and Examples.For example, a part of an embodiment may be replaced by known art, oradded with known art. That is, a part of an embodiment of the inventionmay be combined with known art and modified based on known art, as faras no departing from a technical concept of the invention.

LEGEND

-   -   100 . . . turbine stator blade;    -   101 . . . inner ring side end wall;    -   102 . . . blade part;    -   103 . . . outer ring side end wall;    -   200 . . . gas turbine;    -   210 . . . compression part;    -   220 . . . turbine part;    -   221 . . . turbine nozzle;    -   300 . . . heat exchanger;    -   301 . . . separation layer;    -   302 . . . fin layer; and    -   303 . . . side bar portion.

The invention claimed is:
 1. A product formed of a cobalt based alloy,the cobalt based alloy having a chemical composition comprising: 0.08 to0.25 mass % of carbon; 0.1 mass % or less of boron; 10 to 30 mass % ofchromium; 5 mass % or less of iron, 30 mass % or less of nickel, thetotal amount of the iron and the nickel being 30 mass % or less;tungsten and/or molybdenum, the total amount of the tungsten and themolybdenum being 5 to 12 mass %; at least one of titanium, zirconium,hafnium, vanadium, niobium and tantalum, the total amount of thetitanium, the zirconium, the hafnium, the vanadium, the niobium and thetantalum being 0.5 to 2 mass %; 0.5 mass % or less of silicon; 0.5 mass% or less of manganese; 0.003 to 0.04 mass % of nitrogen; and thebalance being cobalt and impurities, the impurities including 0.5 mass %or less of aluminum, and 0.04 mass % or less of oxygen, wherein theproduct is a polycrystalline body of matrix phase crystal grains, andwherein in the polycrystalline body MC carbide phase grains and M₂₃C₆carbide phase grains are co-precipitated, the MC carbide phase grainsbeing dispersively precipitated in the matrix phase crystal grains at anaverage intergrain distance of 0.13 to 2 μm, the M₂₃C₆ carbide phasegrains being precipitated on grain boundaries of the matrix phasecrystal grains, and wherein the MC carbide phase grains are grains of anMC carbide phase comprising the titanium, the zirconium, the hafnium,the vanadium, the niobium and/or the tantalum; and the M₂₃C₆ carbidephase grains are grains of an M₂₃C₆ carbide phase comprising thechromium, the iron, the tungsten, the molybdenum and/or the manganese.2. The product according to claim 1, wherein: in the case that thechemical composition includes the titanium, the content of titanium is0.01 to 1 mass %; in the case that the chemical composition includes thezirconium, the content of zirconium is 0.05 to 1.5 mass %; in the casethat the chemical composition includes the hafnium, the content ofhafnium is 0.01 to 0.5 mass %; in the case that the chemical compositionincludes the vanadium, the content of vanadium is 0.01 to 0.5 mass %; inthe case that the chemical composition includes the niobium, the contentof niobium is 0.02 to 1 mass %; and in the case that the chemicalcomposition includes the tantalum, the content of tantalum is 0.05 to1.5 mass %.
 3. The product according to claim 1, wherein the productexhibits a creep rupture time of 1,200 hours or more by a creep testunder conditions of a temperature of 850° C. and a stress of 168 MPa. 4.The product according to claim 1, wherein the product is a turbinestator blade, a turbine rotor blade, a turbine combustor nozzle, or aheat exchanger.
 5. The product according to claim 3, wherein the productexhibits a creep rupture time of 1,200 hours or more by a creep testunder conditions of a temperature of 850° C. and a stress of 168 MPa. 6.A method for manufacturing the cobalt based alloy product according toclaim 1, the method comprising: an alloy powder preparation step ofpreparing a cobalt based alloy powder having the chemical compositionaccording to claim 1, the cobalt based alloy powder having a particlesize; a selective laser melting step of forming an additivelymanufactured article, the step comprising alternate repetition of analloy powder bed preparation substep of laying the cobalt based alloypowder such that it forms an alloy powder bed having a predeterminedthickness and a laser melting solidification substep of irradiating apredetermined region of the alloy powder bed with a laser beam tolocally melt and rapidly solidify the cobalt based alloy powder in theregion, the laser beam having an output power and a scanning speed; afirst heat treatment step of subjecting the additively manufacturedarticle to a first heat treatment at a temperature ranging of 750° C. ormore and less than 1,100° C.; and a second heat treatment step ofsubjecting the first heat-treated article to a second heat treatment ata temperature ranging from 600° C. to 1,000° C. but less than thetemperature of the first heat treatment, wherein in the selective lasermelting step, the predetermined thickness of the alloy powder bed h(unit: μm), the output power of the laser beam P (unit: W), and thescanning speed of the laser beam S (unit: mm/s) are controlled tosatisfy the following formulas: “15<h<150” and “67×(P/S)3.5<h<2222×(P/S)+13”.
 7. The method for manufacturing the cobalt basedalloy product according to claim 6, wherein the alloy powder preparationstep comprises an alloy powder classification substep of regulating theparticle size of the cobalt based alloy powder to 5 to 100 μm.